Telephone and pipeline companies, electric, water and gas utilities and others have need for locating and identifying underground lines and subterranean installations to service and maintain them. A single tracing wire is sometimes buried with a utility line wherein the earth is used as a return path. An alternating current signal source is applied between the wire and ground or inductively coupled to the wire when the wire is to be traced. An electromagnetic or "H" field is produced along the wire by the alternating current flow established in the wire. This "H" field is detected by a receiver carried by an operator above the wire to trace the wire. U.S. Pat. No. 4,119,908 discloses a method for tracing utility lines and locating other buried objects wherein a single, buried, tracing wire is provided in the vicinity of the buried utility line and passive markers, which contain a resonant circuit, are disposed adjacent and along side the tracing wire at each point where a particular object or structure is buried. An alternating current signal is applied directly or by induction to tracing wire allowing the tracing wire to be traced as indicated above. A receiver, when swung to and fro laterally of the energized tracing wire, will normally produce an output having a peak-no-peak sequence. The resonant circuit of each of the passive markers, which is inductively coupled to the tracing wire, is tuned to the frequency of the alternating current signal flowing in the tracing wire so that the output of the receiver presents a peak when the receiver reaches a point where a null should be produced to provide a precise location of the passive marker.
U.S. patent application, Ser. No. 153,513, filed Feb. 8, 1988, which is a continuation of U.S. patent application Ser. No. 900,453, filed Aug. 26, 1986, by the inventors which issued as U.S. Pat. No. 4,767,237 on Aug. 30, 1988, discloses the use of more than one insulated conductor which are carried by a marking tape. Two conductors on the marking tape are terminated at one end with the alternating signal source connected between the conductors at the other end to provide a balance circuit as opposed to the single ended circuit that is established when a ground return is used with a single conductor. Passive markers are positioned along and between the two conductors where desired with the resonant circuit of each marker inductively coupled to each of the conductors. The two conductor balanced circuit arrangement causes both the outgoing and return current to inductively energize the passive markers to produce an electromagnetic or "H" field.
The prior art arrangement of U.S. Pat. No. 4,119,908, supra, that has been described, provide for the locating of a buried conductor(s) which may be positioned adjacent to and along a buried utility or supply line, such as a telephone cable, gas or water line, to establish the location of the buried utility line and the point where lateral or branch lines to the main line. A logical extension of such arrangement for establishing the path of a buried utility or supply line that branches from a main line calls for the use of a branch conductor for each branch line which would be connected by ohmic contact with the main tracing conductor. Once the point at which a branch tracing conductor is located, the path of the branch conductor can be determined. This approach requires cutting of the main tracing conductor at each branch point and splicing of branch tracing conductor to the main tracing conductor. Due to the placement of the locating conductors beneath the surface of the earth and the circumstances surrounding such placement, it is difficult and time consuming to provide for a good and long lasting physical or ohmic connection of each branch conductor to the locating conductor for a utility line or the like. All of branch or lateral tracing conductors are energized when a signal is presented to the main tracing conductor causing the "H" field around the main tracing wire to be distorted causing erroneous path information to be generated. In addition, the use of lateral conductors, which make ohmic contact with the conductor for a main utility line, limits the monitoring capability that is provided when a marking tape of the type disclosed in the aforementioned continuation patent application is utilized.